1. Field of the Invention
This invention relates to color laser marking of articles, especially security documents.
2. Description of the Related Art
Articles are laser marked in order to ensure product safety and authenticity. For example, packaging material of pharmaceuticals is laser marked to enable a consumer to verify the genuineness of a product. Laser marked security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various papers or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such articles and security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking an article, a color change is observed by local heating of material in the bulk of the article, while in laser engraving material is removed by ablation.
Today, laser marking employed in the manufacture of security documents consists solely of a “black” laser marking method via the carbonization of a polymer, usually polycarbonate as disclosed in e.g. EP 2181858 A (AGFA). Nevertheless, some modification of articles and security cards remains possible when made by the black laser marking method, since additional carbonization of the polymer can be performed on the articles and security cards.
There has been considerable interest in being able to produce multicolor images through laser marking. It would also be desirable to have a color laser markable article, which can also be “inactivated” after all the information has been marked on the article, meaning that no information can be added or removed from the article.
U.S. Pat. No. 7,158,145 (ORGA SYSTEMS) discloses a three-wavelength system (440, 532 and 660 nm) for applying colored information to a document by means of wavelength-selective bleaching of chromophoric particles in a layer close to the surface. Although a bleached color cannot be restored, modification of the colored information remains possible by extra bleaching.
EP 0174054 A (POLAROID) discloses a heat sensitive element used in a thermal imaging method for forming color images which relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift from colorless to colored, from colored to colorless or from one color to another. EP 0174054 A (POLAROID) suggests using infrared absorbers that absorb radiation at 760 nm, 820 nm and 880 nm. There is however no disclosure how to prevent modification of the color image by additional laser marking, which is also the case for similar imaging methods disclosed in U.S. Pat. No. 4,720,449 (POLAROID), WO 2009/140083 (3M) and U.S. Pat. No. 4,663,518 (POLAROID).
U.S. Pat. No. 5,219,703 (KODAK) discloses a laser-induced thermal dye transfer imaging method wherein the infrared dye absorbs laser radiation and converts it into heat which vaporizes dyes in a dye-donor element and transfers them to a dye-receiver element. After transfer, the infrared dyes which cause an undesirable visual light absorption in the dye-receiver element are bleached by an acid-photogenerating compound formed by infrared or ultraviolet radiation exposure of the dye-receiver element.
A solution to hinder or prevent modification of a color image by additional laser marking would be to use the bleaching of the infrared dye in, for example, the color laser markable article of EP 0174054 A (POLAROID).
However, it was found that bleaching with UV radiation results in direct color formation from the colorless dye without the need for an infrared dye. The alternative bleaching method of using infrared radiation is simply not an option. In the thermal dye transfer imaging method, the color image is transferred and exposure of the dye-receiver element does not result on additional colored dyes being transferred. In a color laser markable article like that of EP 0174054 A (POLAROID), infrared exposure will simply result in undesired additional color formation.
Hence, there remains a need for having a secure color laser marking system, wherein modification by additional color laser marking of laser marked color image is hindered or prevented.